Abstract Alcohol and drug use problems that onset in adolescence are associated with a more severe and persistent course and impairments in multiple domains of psychosocial functioning. A key predictor of loss of control of substance use is the difference in activation between the reward and cognitive control networks. Specifically, the greater the activation of the reward network to drug cues relative to that of the control network, the less control over drug use behavior. During adolescence, maturation of the reward network outpaces that of the control network, resulting in a bias toward risk-taking when in the presence of reward cues. In adolescents but not adults, the presence of peers has been shown to increase risk-taking due to greater activation of the reward network. Adolescents spend more time with peers than adults and deviant peer affiliation is the strongest correlate of alcohol and drug use problems, and drinking alcohol is an especially social activity shared with peers. The combination of these neurodevelopmental and peer influences on the reward network then may be key neurobiological and contextual mechanisms that account for the large increases in alcohol and drug use problems during adolescence. We will examine the development of the neurobiological processes of the reward and cognitive control networks, peer effects on these networks, and how these neurobiological and contextual processes contribute to risk-taking and alcohol and drug use problems in adolescence using a longitudinal fMRI study. We will use an accelerated longitudinal cohort design that covers pre- (10-12 years-old), middle (13-15 years-old), and later (16-18 years-old) adolescence (total N=210), with 1-year and 2-year follow-up assessments. This design will allow us to cover 10 years (10 to 20 years-old) of neurobiological development in half the time. Our protocol will include an assessment of peer presence on reward activation during risking-taking (stoplight task) and reward anticipation and receipt (monetary incentive delay); neurological processes associated with explicit social feedback in the form of acceptance and rejection (chatroom social interaction task); and a basic cognitive control task (stop signal) to assess inhibitory processes. We predict that peer presence will increase reward activation during risk taking and reward receipt, and that these peer effects on reward activation will increase from pre- to middle adolescence. Further, greater reward reactivity and weaker cognitive control will be associated with greater sensitivity to peer influences. Finally, greater reward reactivity, weaker cognitive control, and greater sensitivity to both peer presence and explicit social feedback will be associated with greater alcohol and drug use problems and comorbid externalizing and internalizing problems.